The invention relates to portable radio telephones and in particular to such telephones for communication with base stations in a digital cellular radio telephone system. The invention also relates to a method of operation of such radio telephones, frequently called handsets. The invention is concerned with `locking on` to a digital cellular radio telephone system to allow communication with the system.
System information is typically transmitted over at least one channel at all times for users to obtain the information necessary to select a channel for communication. This may be one or more dummy signals transmitting at all times or just when there are no traffic channels. A handset wishing to lock on to a system must be able to find a channel on which the information required is being transmitted. In digital systems, in order to receive this information the handset must bit synchronise with the signal transmitted.
Every handset will have specific access rights to specific radio telephone systems. In order to set up a call on a channel (that is a combination of a carrier frequency and time slot), the handset must have the right of access to the particular system providing the channel. A handset is provided with one or more Access Right Keys (ARKs) that relate to a particular radio telephone system. The handset is able to lock on to any system that recognises one of the ARKs.
In prior art methods of locking on to a system, the handset assumes a reference frame having the same duration as a frame of the system and which is divided into the requisite number of time slots to provide a complete set of reference time slots. The reference time slots are not synchronised to the time slots of the system being monitored as the handset needs to find a signal being transmitted before synchronisation can occur. The handset monitors the RSSI of each of the reference channels by monitoring each timeslot of a carrier in turn. Once the RSSIs of all the reference channels have been determined the handset selects the reference channel with the highest RSSI, synchronises with the signal and reads the information being transmitted in that channel. The handset is then in a position to determine whether the access rights of the handset allow it to lock on to the system it is now monitoring.
The channel with the highest RSSI is likely to be the channel on which the system is transmitting a dummy bearer or a traffic channel. This is not, however, necessarily the case. A high RSSI could result from noise which may occur because of interference between channels of different systems. The handset will not, therefore, be able to lock on to the system. Even if the highest RSSI does indicate a transmission channel, it is not necessarily a channel on a system to which the handset has access rights. So in the same way that the handset cannot lock onto a noise channel selected as a result of a comparison of the RSSIs, the handset will be unable to lock on to a transmission channel of a system that does not allow the handset access.
If the handset is not able to lock on to the selected channel for any reason a problem arises. Because the reference channels are not synchronised with the transmission time slots of the system, as soon as the handset synchronises with the system to read information on the selected channel, the recorded RSSI information is no longer relevant to the new frame of reference of the handset. If the handset is reading noise it has lost its frame of reference altogether. If the handset is reading a signal from a system to which it is not allowed access, it has adopted the timing of that system. Under either set of circumstances, the handset cannot select a further channel based on the reference information it has previously recorded.
To make a new channel selection all the RSSI information must be gathered afresh. This is both time consuming, and power inefficient. In addition, if there is more than one system in the vicinity of the handset, once the handset has locked on to the one with the strongest signal and found it is not allowed access to that system, it will be difficult to disregard signals resulting from transmissions on channels resulting from that system in order to obtain information on signals from other systems that might also be in the vicinity.